Heme oxygenase-1 mediates the protective effects of rapamycin in monocrotaline-induced pulmonary hypertension

Abstract

Rapamycin inhibits the development and progression of vascular disease. We previously showed that rapamycin induces the cytoprotective protein, heme oxygenase-1 (HO-1), and more importantly, chemically inhibiting HO-1 blocked the antiproliferative actions of rapamycin. In this study, we evaluated whether HO-1 is required for the vascular protective effects of rapamycin in vivo using a rat monocrotaline-induced pulmonary hypertension model. Rats were exposed to monocrotaline with or without rapamycin and HO activity was altered using the chemical inhibitor, tin protoporphyrin or the inducer, cobalt protoporphyrin. We also evaluated possible mechanisms of rapamycin-dependent induction of HO-1, and how HO-1 mediates growth factor-dependent antiproliferative actions of rapamycin. Proliferation and cell cycle progression were examined in smooth muscle cells derived from both wild-type and HO-1 knockout (HO-1 À/À ) mice in response to growth factors and rapamycin. Similar to our previous findings in vitro, rapamycin induced HO-1 in rat lung. Rapamycin also inhibited the development of monocrotaline-induced pulmonary hypertension, and this protective effect was blocked with the addition of tin protoporphyrin. In addition, treatment with cobalt protoporphyrin resulted in a substantial protection in this model of pulmonary hypertension. Rapamycin induction of HO-1 was dependent upon a transcriptional event; however, it was not mediated through an altered redox state or mammalian targets of rapamycin inhibition. Unlike wild-type cells, the growth of HO-1 À/À mouse aortic smooth muscle cells was not inhibited or cell cycle arrested in G1 in response to rapamycin. This study demonstrates that HO-1 is critical for the antiproliferative and vascular protective effects of rapamycin in vitro and in vivo in monocrotaline-induced pulmonary hypertension. Laboratory Investigation (2006Investigation ( ) 86, 62-71. doi:10.1038 published online 31 October 2005 Keywords: animal model; aorta; heme oxygenase; monocrotaline; pulmonary hypertension; rapamycinThe fibroproliferative response to injury is a pathologic property of vascular diseases, such as pulmonary hypertension.1-3 Pathologic changes observed in pulmonary arterial hypertension (PAH) include medial hypertrophy, adventitial thickening, and abnormal growth of endothelial cells. A limitation of current therapies for PAH is that they are mostly aimed at vasodilatation, rather than against the fibroproliferative response.

Effects of erythropoietin on advanced pulmonary vascular remodelling

“…Increasing HO-1 in this model is beneficial [27]. Therefore, HO-1 may have a central role in the effects of EPO in pulmonary hypertension that warrants further investigation.…”

Heme Oxygenase-1

“…In addition, inhibition of HO activity by tin protoporphyrin resulted in a loss of the antiproliferative activity of rapamycin, 61 and smooth muscle cells from HO-1 Ϫ/Ϫ mice were refractory to growth inhibition by rapamycin. 62 Similarly, rapamycin was recently reported to induce HO-1 in the lungs of rats and to inhibit the development of monocrotaline-induced pulmonary hypertension, with the protective effect being blocked by co-treatment of the animals with tin protoporphyrin. 62 Collectively, these findings suggest that the antiproliferative action of rapamycin may be modulated, at least in part, by its actions on HO-1.…”

“…62 Similarly, rapamycin was recently reported to induce HO-1 in the lungs of rats and to inhibit the development of monocrotaline-induced pulmonary hypertension, with the protective effect being blocked by co-treatment of the animals with tin protoporphyrin. 62 Collectively, these findings suggest that the antiproliferative action of rapamycin may be modulated, at least in part, by its actions on HO-1. Similar to rapamycin, paclitaxel, which induces apoptosis, interferes with normal function of microtubule growth, and is used to treat in-stent restenosis, induces HO-1 in vascular smooth muscle cells.…”

The Role of Heme Oxygenase-1 in Pulmonary Disease

“…Endogenous overexpression of HO-1 using agonists of HO-1 prevents the development of hypoxia-induced pulmonary hypertension and vascular remodeling in the rat (13), while selective overexpression of HO-1 in the pulmonary epithelium of mice attenuates hypoxia-induced pulmonary inflammation and pulmonary hypertension (14). Similarly, in a monocrotaline model of pulmonary hypertension, HO-1 mediates the protective effects of rapamycin (39) and inhibition of HO-1 exacerbates pulmonary inflammation and RV hypertrophy (40).…”